Controlling chemistry by geometry in nanoscale systems.

Scientific literature dealing with the rates, mechanisms, and thermodynamic properties of chemical reactions in condensed media almost exclusively assumes that reactions take place in volumes that do not change over time. The reaction volumes are compact (such as a sphere, a cube, or a cylinder) and do not vary in shape. In this review article, we discuss two important systems at small length scales (approximately 10 nm to 5 microm), in which these basic assumptions are violated. The first system exists in cell biology and is represented by the tiniest functional components (i.e., single cells, organelles, and other physically delineated cellular microenvironments). The second system comprises nanofluidic devices, in particular devices made from soft-matter materials such as lipid nanotube-vesicle networks. In these two systems, transport, mixing, and shape changes can be achieved at or very close to thermal energy levels. In further contrast to macroscopic systems, mixing by diffusion is extremely efficient, and kinetics can be controlled by shape and volume changes.